Elena Kartashova Scott McCallum Quantifier Elimination for Approximate Factorization of Linear Partial Differential Operators. 106-115 2007 conf/mkm/2007 Calculemus/MKM http://dx.doi.org/10.1007/978-3-540-73086-6_9 db/conf/mkm/mkm2007.html#KartashovaM07